Antennas



June 21, 1960 D. BALDWIN ANTENNAS 2 Sheets-Sheet 1 Filed Feb. 25, 1957FIG. 2

FIG. 3

FIG. I PRIOR ART FIG. 4

V/A/ V/ /////////////////a INVENTOR LEROY D. BALDWIN lob I; I I I I' BY(Z1 ATTORNEY FIG.6

June 21, 1960 Filed Feb.

VOLTAGE STANDING WAVE RATIO L. D. BALDWIN ANTENNAS 25, 1957 2Sheets-Sheet 2 H PLANE RADIATION PATERN OF QUARTER WAVE SLOT ANTENNADECIBELS PROBE DETAIL INVENTOR. FREQUENCY (MC) LEROY o. BALDWIN FIG. 8

A AT RNEY United States, Patent 4 ANTENNAS Leroy D. Baldwin, Rochester,N.Y., assignor to General Dynamics Corporation, Rochester, N.Y., acorporation of Delaware Filed Feb. 25, 1957, Ser. No. 642,039

3 Claims. (Cl. 343-767) This invention relates to slot antennas and isparticularly directed to means for shortening the length of the slotwithout loss of radiating efiiciency and, at the same time, improvingthe directional characteristics of the antenna.

The slot antenna of the type treated here may be mathematically analyzedby an elongated opening or slot in a plateof extended area and goodelectrical conductivity, as shown in Figure lof the accompanyingdrawing. The slot is analogous to the arms of a'dipole antenna, each armof which is one-quarter ofthe operating wave length. This requires thedipole or the slot to be at least one-half wave length long. Byimpressing the high frequency voltage on opposite sides of the slotmidway between its ends, maximum impedance can be presented to thetransmission line feeding the energy to be radiated. Since the highfrequency current disperses into the ground plate, the mean electricalpath between the points to which the energy isapplied becomes reasonablylong and radiation reasonably efiicient. Heretofore, it has not beenpossible to construct the slot less than the-quarter wave length oneither side of the transmission line else the line becomesshort-circuited and the antenna fails.

For various electrical and mechanical reasons it is desirable "that slot'lengthsless thanone-half wave length be employed. For exampleflthe sizeof the ground plate containing the slot may be limited in size formechanical reasons and the operating frequency may be low, 'thus makingit unfeasible to use the half wave length slot. In cases where broadbands of frequencies. are to be radiated or received, the quarter wavedimension of the slot must be taken for the lowest frequency of theband.

The object of this invention is' to provide? an improved slot antennaless than one-half wave length long yet having eflicient radiatingcharacteristics and good directionability over a broad band offrequencies.

The object of this invention is attained by forming a slot in a groundplate of extended area inward from the edge of the plate a distancecorresponding to about onequarter of a wave length at the lowestoperating frequency. Means is provided for energizing the slot near itsopen end. A housing may be placed over the slot to prevent its radiationfrom one plane of the slot, and means is provided for establishing ahigh impedance between the sides of the slot adjacent said open end andfor obviating short-circuiting effects of the housing, said meanscomprising resonant stubs on either side of the slot within the housingto present a high impedance load to the slot so that high frequencyenergy may be transferred efiiciently to the open end of the slot.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the specific embodiments of theinvention described in the following specification and shown in theaccompanying drawing in which:

Figure 1 is a plan view of a conventional slot antenna of the prior art,

Figure 2 is a plan view of a slot antenna embodying this invention,

2,942,263 Patented June 21, 1960 one slot antenna'emvention,

Figure 7 is an enlarged detail sectional taken on line 7--7 of Figure 6,

Figure 8 is a graph of the standing wave ratio of a broad band antennaof this invention, and Figure 9 is a polar diagram of the field strengthquarter wave antenna of this invention. According to this invention, theground plate 1, Figure 2, is slotted inwardly as at 2 from one edge ofthe ground plate, the length of the slot being about one-quarter of awave length in extent as contrasted to the one-half wave length forslots of the prior art; such as shown in Figure 1. The transmission line3 maybe thus coupled directly to opposite sides of the slot adjacent theedge of the plate. The transmission line may comprise the two-wire lineof Figure 2, or the coaxial line for Figure 3, or the specialtransmission line of Figures 4, 5, and 6. Further,

of the the two sides of the line either may be attached directly toopposite sides of the slot as in Figure 2, or coupled to the spacetherebetween by an irradiating probe in the slot as suggested at 5 inFigure 3. it has been found that such a slot will radiateperpendicularly to either side to the ground plate 1, and thatnegligible radiation occurs from the end of the slot in the plane of theground plate 1.

A conventional, cup-shaped housing cannot be .placed over the slot toform a cavity and to confine radiation to one side of the ground platebecause the housing would short-circuit the transmission line and reducethe impedance of the open end of the slot to zero. According to animportant feature of this invention, the housing 10' is placed over theslot and is attached to the ground plate 1, but the housing isopen-ended at the openend of I the'slot and the side panels of thehousing are spaced outwardly from the side edges of the slot.Conveniently, 'three flanges 10a, 10b, 100 are formed on the edges ofthe housing and'are soldered, brazed, or otherwise electrically andmechanically attached to the ground plate. Preferably, the inside lengthof'the housing 'is the same as the length of the slot. Such ahousingwill prevent radiation from the slot toward the rear of thegroundplate, but, as thus far described, will cause strong radiation from theend of the slot in the plane of the ground plate. I

are placed along either side of the slot and are extended into thehousing a distance which is a significant fraction of a' 'wave length.That is, the foldedcav'ities formed by the housing and flanges are soproportioned as to produce an impedance at the slot, which impedance ismatched to the transmission line impedance. Ideally, electric wavescreated between the edges of the slot by the transmission line will seetwo stubs or cavities exactly onequarter wave length long when measuredfrom this slot toward the bottom wall of the housing, around the outeredges of the flanges 11 and 12, andrhence to the shortcircuited end ofthe stub between the flanges and housing at the backof the groundplate 1. This quarter wave dimension must, however, be compromised tominimize radiation from the ends of the cavities at the open'end of thehousing. Extensive experimentation with the cavities of the type shownin Figure 3 shows that a good impedance match and minimum end radiationcan be achieved for broad band operation when the mean electrical lengthof the stubs are each less than one-quarter of the longest wave of theoperating band. Hence, the flanges 11 and 3 12 extend from-the-edges'ofthe slot into the housing a distance-corresponding to, or less than,one-eighth of the longest wave of the operating band.

Figures 4, 5 and 6 show the top, edge, and bottom views,-respectively,'of one particularly effective antenna of this invention. In thisembodiment, the transmission line '-is printedon one side of a thinsheet 19 of a high grade insulating material.- The printed transmissionline comprises the printed strip terminating at one end in a radiatingprobe 15a of measured area. 'To the other side of the insulating sheetis attached a thin foil 17 of extended area: compared to'the thicknessof the insulating sheet and to the width-of the printed -transmissionline 15. 50 called microstrip" laminates are commercially obtainable,comprising a thin sheet of Tefion-impregnated'fiberglass board-clad onboth sides with copper foil. Insconstructing the-antenna of thisinvention, the foil on one side is coated with a photo-resist and isselectively irradiated by a strong light through a ncgative of the pattern ofthe printed circuitry desired. In this case, the negative isprepared to show the exact outline of the transmission line 15 and itsprobe end 15a. Then the 4 Figure 9, the principal lobe of the antennapattern being outward from one side of the quarter wave slot with littleradiation from the rear or open end of the housing.

Many mechanical modifications may be made in the specific structureshown without departing from the scope of the invention as defined inthe appended claims.

What is claimed is:

l. A slot-type antenna comprising a ground plate, the plate having awindow therethrough adjacent one edge of the plate, said windowextending inward from said edge a distance approximately one-quarter ofa wave length at the lowest operating frequency of the antenna, meansfor activating said window with energy of said operating frequency, ahousing attached -to-one side' of said plate and enclosing said windowto confine radiation from the other side of said plate, said housingbeing open at the end adjacent said edge to prevent short-circuitingsaid window at said edge, and means to prevent radiation from the openend of said housing comprising flanges along the side edges of saidwindow and extending into the cavity of said housing toformshort-circuited stubs along either side of the window.

unwanted portionof the foil is selectively removed in anetchingsolution. The window 16 in the ground plate 17, .on the otherside of the sheet,.as.best shown Figure 7, is removed also by theetching :technique if desired, :the removed portion being at'least aslarge as the desired slot. The window .16 may be :formed :entirelywithin the boundaries of the ground plate, as shown, or along one edgethereof. 1 he window is formed .to underlie, the antenna-probe 15a and.is formed to the exact size and shape of .the desired slot. Since thesandwich comprising the insulating sheet, ground plate, transmissionline, and probe are mechanically bonded together and are .incapable ofrelative movement, the precise position ofithe window slot with respectto the probe .is easily established and reproducible in manufacture. Ithas been found-desirabe for good impedance matching and forefiicientradiation to extend the area of the probe, as shown :atlSa inFigure 4. vThe printed circuit board is then bolted or otherwisefastened to the metal chassis 18 in which the slot 16a has been cut. Theedges of the window 16 of the printed circuit board are made to registerwith the edges of the slot 16a in the chassis. The metal angles 11 and12 are soldered, brazed, or otherwise affixed along the edges of window16a, and the housing 10 conveniently formed from sheet metal and withflanges 10a. 10b and 10c, is placed over the angles as shown andfastened to the chassis. The depth and width of the housing 10is'selected -to provide the stub cavities around the angles 11 and .12.The open end .ofthehou'sing .ispIaced flush with the edge .of the groundplate. The details of the slot structure are exaggerated .in Figure 7 tobetter show the arrangement of parts.

Figure 8 is adiagram of the voltage standing wave ratio of'the quarterwave slot antenna of Figures 4, 5 and 6 for variousfrequencies, when theprobe and .slothas the specific dimensions shown in Figure 8..Alteration of any of the dimensions will, of ocurse, alter differentsmall portions of the voltage wave pattern but will not materiallychange the broad outline of the wave pattern. Where the dimensions ofthe probe end of the printed transmission line are of the specificvalues shown in .Figure 8, it is seen that the antenna has a remarkablebroad band characteristic, the response being substantially uniform from1800 to 3600 megacycles. The H plane radiation pattern of the quarterwave slot antenna for frequencies of the order of 2000 megacycles isshown in 2. In a unitary slot antenna assembly, aground plate ofextended surface area, a'slotin said plate, .a transmission line forenergizing said slot at an operating 'frequency, means for matching theimpedance of said slot at said frequency :to the impedance of saidtransmission line comprising a rectangular-type housing affixed to saidplate along lines "parallel to and spaced from the side edges of .saidslot, .flangesalong and coextensive with the side edges of said .slotand extending into said housing to form folded cavities, said cavitiesbeing open at .the :slot end andshort-circuited at the other end by saidplate.

3. In combination in an antenna structure, a sheet of insulatingmaterialzof good insulating properties at microwave frequencies, a striptransmission 'lineaffixed to one surface of said sheet, a ground plateof extended area against the othersurface of ,said sheet, an elongatedslot in said plate, said transmission line terminatingopposite saidslot, said slot being about one-quarter wave length long and beingcfiectively open circuited atone end for the operating frequency,-fianges. along the side edges of said slot, and a housing attached tosaid ground plate and enclosing" said slot and flanges, said flangeshaving such width and said housing being so spaced from said flanges asto form elongated impedance matching stubs along opposite edges ofsaid-slot;

ReferenceslCited in the file-of this patent UNITED STATES PATENTS OTHERREFERENCES Journal of Institution of ElectricalEngineers, Bailey: SlotFeeders and Slot Aerials," vol. 93, part .IIIA, No.

4, May 1946, page 617.

v The proceedingsof the Institution of Electrical En ineers (British),vol. 97, ,part llL No. 5.0, November 1950, pp. 414-417, page 415.

